Both the production and function of immunoglobulin E (IgE) molecules are intimately dependent on IgE-binding proteins. One type of IgE-binding protein includes the cell surface receptors of mast cells, basophils, lymphocytes and other cell types (Ishizaka, T. and Ishizaka, K., Prog. Allergy, 19:60-121, 1975; Froese, A., CRC Crit. Rev. Immunol., 1:79-130, 1980). The receptors on mast cells and basophils are responsible for IgE-mediated immediate hypersensitivity reactions (Ishizaka, K. and Ishizaka, T., Immunol. Rev., 41:109-148, 1978; Metzger, H., Alearaz, G., Hohman, R., Kinet, J.-P., Pribluda; V. and Quarto, R., Annu. Rev. Immunol., 4:419-470, 1986), while those on lymphocytes play important roles in the regulation of IgE antibody responses (Spiegelberg, H. L., Adv. Immunol., 35:61-88, 1984; Ishizaka, K., Annu. Rev. Immunol., 2:159-182, 1984). Another type of IgE-binding protein includes the lymphokines which function in either potentiating or suppressing IgE antibody production (Spiegelberg, H. L., Adv. Immunol., 35:61-88, 1984). In order to further understand the IgE system, it is important to establish the structural relatedness of these proteins, the structure-function relationship of each of these proteins, and the regulation of their gene expression. To this end, this invention's primary objective is to initiate molecular cloning of DNA for these relevant proteins.
Rat basophilic leukemia (RBL) are cells which have been used extensively to study a high affinity IgE receptor present on basophils and mast cells and from which IgE-binding proteins have already been isolated and characterized (reviewed in Ishizaka, T. and Ishizaka, K., Prog. Allergy, 19:60-121, 1975; Metzger, H., Alearaz, G., Hohman, R., Kinet, J.-P., Pribluda, V. and Quarto, R., Annu. Rev. Immunol., 4:419-470, 1986; Katz, D. H. and Marcelletti, J. F., Prog. Immunol. Proc. Int. Congr. Immunol., 5:465-482, 1983). One well-studied IgE-binding protein is a glycoprotein of M.sub.r =55,000 which has been studied in a number of laboratories (Metzger, H., Kinet, J.-P., Perez, Montfort, R., Rivnay, B. and Wank, S. A., Prog. Immunol. Proc. Int. Congr. Immunol., 5:493-501, 1983; Hempstead, B. L., Parker, C. W. and Kulczycki, A., Jr., J. Biol. Chem., 256:10717-10723, 1981; Kulcycki, A., Jr. and Parker, C. W., J. Biol. Chem., 254:3187-3193, 1979). It has been definitively established that this protein is expressed on the surface of RBL cells, mast cells and basophils, and is responsible for the high-affinity IgE-binding property of these cells.
The biochemistry of the high-affinity IgE receptor became somewhat complex when a multi-subunit nature of the receptor was revealed by the identification of other protein components in the efforts to purify the receptor from RBL cells. One group of scientists has isolated two additional proteins, M.sub.r =33,000 (Hempstead, B. L., Parker, C. W. and Kulczycki, A., Jr., Proc. Natl. Acad. Sci. U.S.A., 80:3050-3053, 1983; Hempstead, B. L., Kulczycki, A., Jr., and Parker, C. W., Biochem. Biophys. Res. Commun., 98:815-822, 1981) and M.sub.r =10,000 (Holowka, D. and Baird, B., J. Biol. Chem., 259:3720-3728, 1984), in the affinity purification of IgE-receptor complexes. They also demonstrated that these two proteins could be chemically cross-linked to the M.sub.r =approximately 55,000 glycoprotein by treating RBL cells or cell lysates with bifunctional cross-linking reagents, and therefore designated them as the .beta. and .gamma. subunits, respectively, of the high-affinity IgE receptor, with the M.sub.r =approximately 55,000 glycoprotein being designated as the .alpha. subunit.
Holowka et al., supra, disclose a polypeptide of molecular weight of 30,000-35,000 which is complexed with a 55,000 molecular weight glycoprotein (.alpha. subunit) and two other smaller polypeptides (.gamma. subunits) to form the high-affinity receptor of immunoglobulin E (IgE) of RBL cells identified as the .beta. subunit of the IgE receptor. The multiple subunit configuration of the IgE receptor has been known for some time, but the exact nature of each subunit and what role it plays specifically in the receptor complex is just now being elucidated. However, it is known that the binding site for IgE is the 55,000 m.w. .alpha. subunit and that it is the only subunit or chain that binds IgE. The exact role of the .beta. subunit is not known, but it is known that it does not bind IgE independently. The 31,000 m.w. protein described herein that binds to IgE (i.e. IgE-Sepharose 4B) is, accordingly, not the .beta. subunit of Holowka et al. In addition, a subsequent paper (Metzger, H., Alearaz, G., Hohman, R., Kinet, J.-P., Pribludo, V. and Quarto, R., Annu. Rev. Immunol., 35: 61-88, 1986) reports that the beta subunit does not contain the number of methionine residues initially considered possible by Holowka et al.
Another group of scientists has isolated a M.sub.r =30,000-33,000 protein in addition to the M.sub.r =55,000 glycoprotein, in the purification of RBL IgE receptor by repetitive affinity chromatography with IgE immunoadsorbent (Liu, F.-T. and Orida, N., J. Biol. Chem., 259:10649-10652, 1984; Liu, F.-T., Albrandt, K., Mendel, E., Kulczycki, A., Jr., and Orida, N. K., Proc. Natl. Acad. Sci. U.S.A., 82:4100-4104, 1985; U.S. patent application Ser. No. 06/726,250; Kulczycki, A., Jr., Isersky, C. and Metzger, H., J. Esp. Med., 139:600-616, 1974). Although this protein shares certain similarities with the protein designated as the .beta. subunit, their exact relationship remains to be firmly established. Furthermore, it remained to be determined whether the isolation of either protein can be attributed to its ability to bind IgE independently. More recently, proteins of M.sub.r =30,000-33,000 which are distinct from the .beta. subunit but which possess an IgE-binding property have been identified in RBL cells (Holowka, D. and Baird, B., J. Biol. Chem., 259:3720-3728, 1984).